1. Technical Field
The present invention relates to a method for producing a particulate water-absorbing agent composed principally of a water-absorbing resin.
2. Background Art
In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, as a water-absorbent agent, in view of absorbing a body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer and a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, and, it is generally used in a water-absorbing agent after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of about 200 to 800 μm, is used widely. The particulate water-absorbing agent is produced via many steps. These steps may include the polymerization step, the drying step, the pulverization step, the classification step, the surface cross-linking step, and the like.
In producing the particulate water-absorbing agent, enhancement of various properties are considered. A production step has been studied with an object of enhancing various properties. For example, US-A-2003/087983 (Patent Document 1) discloses a production method for removing metallic foreign substances by a magnetic ray irradiation step. This production method can suppress deterioration of a water-absorbing resin effectively.
In a production process of the particulate water-absorbing agent, moisture evaporates from a polymer gel or the like. Humidity containing this steam can spread to the whole area of a production facility via a production apparatus or a pipeline of each step. This humidity can generate dew condensation in the inside of the production apparatus or the pipeline. When the particulate water-absorbing resin during the production process absorbs humidity or moisture, the particulate water-absorbing resins themselves may aggregate or the particulate water-absorbing resin tends to attach to the inner surface of the production apparatus or the pipeline, which disturbs stable production. In addition to this, moisture absorption by the particulate water-absorbing resin in the production step may decrease quality of the particulate water-absorbing agent obtained finally.
In addition, in producing the particulate water-absorbing agent, formation of the fine powders (for example, the fine powders with a particle diameter of below 150 μm) of the water-absorbing resin is accompanied. If the fine powders are contained in disposable diapers and the like, they clog absorbent articles composed of the particulate water-absorbing agent, which causes decrease in liquid permeability of the absorbent articles. In addition, the fine powders influence surface cross-linking effect of the particulate water-absorbing agent, and even if surface cross-linking is performed to the particulate water-absorbing agent containing the fine powders, there can be the case where various properties such as liquid permeability, absorbency against pressure are not enhanced sufficiently.
Amount of the fine powders generating in producing the particulate water-absorbing agent can become several % by weight to ten and several % by weight, and in certain cases, as many as several tens % by weight of solid content of the polymer gel obtained in the polymerization step, that is, a dry substance or a dried pulverized substance. Because disposal of the fine powders accompanies generation of disposal cost of the fine powders in addition to decrease in yield of the particulate water-absorbing agent, it incurs increase in production cost of the particulate water-absorbing agent. Because the fine powders are inferior in properties, it is difficult to utilize the fine powders to other applications without disposal. Because demand for the fine powders is generally low, it is also difficult to sell the fine powders.
Accordingly, as a method for obtaining a cheap particulate water-absorbing agent containing less fine powders, many methods for reusing the fine powders, that is, recycling methods for the fine powders have been proposed. As the recycling methods for the fine powders, there are (I) a method for mixing the fine powders as they are into an aqueous solution containing a monomer before polymerization to polymerize (refer to Patent Document 2), (II) a method for mixing the fine powders as they are into a gel in the way of polymerization to polymerize (refer to Patent Documents 3 and 4), (III) a method for mixing the fine powders as they are into a polymer gel obtained by polymerization, (IV) a method for mixing a large particle obtained by granulating the fine powders in aqueous liquid into a polymer gel obtained by polymerization (refer to Patent Document 5) and the like. Among these, in the methods (I) to (III), the fine powders are used as they are. In these methods, uniform mixing of the fine powders is difficult, and further the fine powders absorb a monomer, water or the like and thus could decrease performance thereof. From this view point, a method (IV) is considered preferable, which utilizes granulated particles composed of a plurality of the fine powders. In the method (IV), the granulated particles (secondary particles) as large particles are formed by binding the fine powders (single particles) themselves generally by use of water as a binder. Because the aqueous liquid as water is contained in the granulated particles, it is necessary to dry the granulated particles. In addition, as an application of the method (IV), it is also considered a method for making particles by pulverizing the granulated gels (tertiaryparticles) obtained by further aggregating the granulated particles themselves composed of a plurality of the fine powders. However, this method results in containing a large quantity of water by the granulated gel, and therefore consumes a large quantity of energy for drying the granulated gel. Such a method cannot be said to be a preferable method due to increase in production cost.
The production method for the particulate water-absorbing agent usually includes the step (surface cross-linking step) for cross-linking the neighborhood of the surface of the water-absorbing resin by adding a surface cross-linking agent to the water-absorbing resin and heating. This step is apt to be accompanied with the step for cooling the particulate water-absorbing agent thus heated. In this cooling step, there may be the case where a method for removing the fine powders of the water-absorbing resin and/or a part of the remaining surface cross-linking agent, while cooling the particulate water-absorbing agent thus heated by the air generated by reduced pressure (refer to Patent Document 6). In this method, surface cross-linking is performed in a state that the fine powders generated in the step before this step are contained as they are. Therefore, the remaining fine powders cannot be removed, and thus there is a problem that the remaining fine powders deteriorate work environment. Because the fine powders deteriorate mixing property of the water-absorbing resin with the surface cross-linking agent, there is also a problem that the neighborhood of the surface of the water-absorbing resin is not cross-linked sufficiently. Further, action of a part of the surface cross-linking agent makes granulation of the remaining fine powders, and the fine powders granulated are crushed in the subsequent steps or in producing an water-absorbing material (for example, in producing absorbent articles such as disposable diapers and the like), and the fine powders thus crushed fly as dust, and thus there are also worry about causing a problem of deterioration of work environment, and a problem that the fine powders thus crushed obstruct performance (for example, liquid permeability) of the absorbent articles.
In addition, with enhancement of performance of the particulate water-absorbing agent, enhancement of properties of the water-absorbing resin, such as absorbency against non-pressure (CRC), absorbency against pressure (AAP) and saline flow conductivity (SFC) to be described later, has been required. In order to respond to this request, a firm surface cross-link structure is introduced to the surface of the water-absorbing resin, or many additives (for example, water-insoluble fine particle and the like) are used. However, such steps take a long time process in many cases. Therefore, the water-absorbing resin particles are destroyed to incur by-production of the fine powders. That is, requiring of high property to the water-absorbing resin results in increasing the fine powders in the water-absorbing agent obtained.